Color-plus-clear coating systems involving the application of a colored or pigmented basecoat to a substrate followed by application of a transparent or clearcoat over the basecoat have become increasingly popular as original finishes for a number of consumer products including, for example, automotive vehicles. The color-plus-clearcoating systems have outstanding appearance properties such as gloss and distinctness of image, due in large part to the clearcoat. Such color-plus-clearcoating systems have become popular for use with automotive vehicles, aerospace applications, floor coverings such as ceramic tiles and wood flooring, packaging coatings and the like.
Topcoat coating compositions, particularly those used to form the transparent clearcoat in color-plus-clearcoating systems for automotive applications, are subject to defects that occur during the assembly process as well as damage from numerous environmental elements. Such defects during the assembly process include paint defects in the application or curing of the basecoat or the clearcoat. Damaging environmental elements include acidic precipitation, exposure to ultraviolet radiation from sunlight, high relative humidity and high temperatures, defects due to contact with objects causing scratching of the coated surface, and defects due to impact with small, hard objects resulting in chipping of the coating surface.
Further, elastomeric automotive parts and accessories, for example, elastomeric bumpers and body side moldings, are typically coated “off site” and shipped to automobile assembly plants. The coating compositions applied to such elastomeric substrates are typically formulated to be very flexible so the coating can bend or flex with the substrate without cracking. To achieve the requisite flexibility, coating compositions for use on elastomeric substrates often are formulated to produce coatings with lower crosslink densities or to include flexibilizing adjuvants which act to lower the overall film glass transition temperature (Tg). While acceptable flexibility properties can be achieved with these formulating techniques, they also can result in softer films that are susceptible to scratching. Consequently, great expense and care must be taken to package the coated parts to prevent scratching of the coated surfaces during shipping to automobile assembly plants.
U.S. Pat. No. 6,235,858 B1 discloses carbamate and/or urea functional polymers for use in coating compositions, especially clear coating compositions for color-plus-clear coating systems. Such polymers provide coatings with good resistance to damage caused by acidic precipitation.
U.S. Pat. No. 5,853,809 discloses clearcoats in color-plus-clear systems which have improved scratch resistance due to the inclusion in the coating composition of inorganic particles such as colloidal silicas which have been surface modified with a reactive coupling agent via covalent bonding.
A number of patents disclose the use of a surface active material, for example, a polysiloxane, in coating compositions to improve mar-resistance of the cured coatings. U.S. Pat. Nos. 5,939,491 and 6,225,434B1 disclose coating compositions comprising organic polysiloxanes having reactive functional groups. These polysiloxanes provide coatings with improved mar and scratch resistance.
A number of patents disclose the use of boric acid in polymeric compositions. For example, U.S. Pat. Nos. 5,951,747 and 6,059,867 discloses the use of boric acid and borates in conjunction with a succinate in non-chromate, corrosion-inhibiting coating compositions for improved adhesion to metallic surfaces. Such compositions further include inhibitors such as phosphates, phosphosilicates, silicates, titanates, and zinc salts. U.S. Pat. No. 4,832,990 discloses a process for improving adhesion of polyolefins to metal substrates comprising mechanical cleaning of the metal surface, treating the metal surface with a water-alcohol solution containing an alkoxysilane and boric acid, thermally treating the acid treated substrate, and subsequently treating the substrate with a polyolefin-based composition comprising zeolites and carbon black pigment. U.S. Pat. No. 5,073,455 discloses a thermoplastic laminated film which has improved adhesion to hydrophilic polymers, hydrophobic polymers and inorganic substances. The film comprise a base film of thermoplastic resin and a layer formed on the base film comprising a composition of one or more of water-soluble resins, water emulsified resins and water-dispersible resins, and an organic boron polymer or a mixture composed of an organic boron polymer and vinyl alcohol.
Other multi-layer composite coatings are commonplace in modern coating lines. For example, a typical automotive coating system can include the sequential application of an electrodeposition primer, a primer-surfacer, a color enhancing base coat, and a transparent top coat. In some instances, the electrodeposition primer is applied over a mill-applied weldable, thermosetting coating which has been applied to the coiled steel metal substrate from which the automobile body (or body parts, such as fenders, doors and hoods) has been formed. Also, adhesive coatings, for example, windshield adhesives, trim and molding adhesives and structural adhesives are sometimes applied to the cured top coats where necessary. Due to these multi-layer composite coating processes, it is necessary that the previously applied coating layer have excellent intercoat or interlayer adhesion to the subsequently applied coating layer(s).
Although the aforementioned coating compositions exhibit improvements for acid etch resistance and mar and scratch resistance, such compositions may not be readily recoatable. That is, when a subsequent coating is applied to the cured mar and scratch resistant coating composition, the intercoat adhesion between the cured coating and the subsequently applied coating can be quite poor.
For example, as mentioned above, on most vehicle coating lines the vehicle body is first given a corrosion inhibitive electrodepositable primer coating commonly formed from a cationic electrodepositable coating composition. This electrodeposition primer is fully cured and, a primer-surfacer is typically applied to the cured electrodeposition primer. The primer-surfacer serves to enhance chip resistance of subsequently applied top coatings as well as to ensure good appearance of the top coatings. The electrodepositable primer must have excellent interlayer, i.e., intercoat, adhesion to the subsequently applied primer-surfacer coating. The top coats, which can include a monocoats as well as a color-plus-clear coating system, are then applied to the cured primer-surfacer coating. While most top coats have excellent intercoat adhesion to the primer-surfacer coating, some top coating compositions inherently may exhibit intercoat adhesion problems with some primer-surfacer coatings.
Also, due to the resultant cost-savings, there is recent interest in the automotive coatings market in eliminating the primer-surfacer step altogether. That is, the top coats can be directly applied to the cured electrodeposition primer. In such modified coating processes, the electrodeposition primer is required to meet stringent durability and appearance specifications. Moreover, the cured electrodepositable primer must have excellent intercoat adhesion to the subsequently applied top coats (either monocoats or color coats of a color-plus-clear system).
On commercial automobile coating lines during application of the coating system, certain portions of the line can experience occasional process problems, for example, clearcoat applicator malfunctions, or curing oven faults where temperatures are out of specification. While the color coat typically is “flash cured” to drive off solvent, but not fully cure the coating, once the clear coating has been applied, the color-plus-clear coating system typically is given a full cure (e.g., 250° F. for 20 minutes) to simultaneously cure both the base coat and the top coat. In instances where the clear coat application system is malfunctioning, the auto body with the applied color coat will continue through the clear coat applicator station and into the clear coat curing oven, thereby fully curing the color coat. If this occurs, some automobile manufacturers elect to reapply the color coat over the fully cured color coat prior to application of the clearcoat. In such situations, the fully cured color coat can have poor intercoat adhesion with the subsequently applied color coat, even though the compositions may be the same.
Also, windshields and other items such as trim moldings typically are affixed to the body of a vehicle with an adhesive material, typically a moisture-cured material containing isocyanate group-containing polymers. Motor Vehicle Safety Standards (MVSS) require that these adhesives have complete adhesion to both the windshield and the coated substrate to which they are applied. Similar adhesive compositions can be used as structural adhesives as well. Such adhesives, for example, are commercially available from Essex Specialty Products, Inc. of Auburn Hills, Mich. These adhesive products adhere well to many cured top coating compositions used to coat vehicles such as automobiles. It is known, however, that these adhesive materials often do not completely adhere to some top coats, for example, those formed from coating compositions based on carbamate and/or urea containing polymers. This necessitates the application of a primer coating to the cured carbamate and/or urea-based top coatings prior to application of the windshield adhesive to ensure compliance with the aforementioned Motor Vehicle Safety Standards. Such primer coatings are typically based on moisture-curable polymers similar to those comprising the adhesive. Use of such primer coatings has proven to be effective, but primer coating application adds an additional and expensive step to the windshield and/or trim installation processes.
Moreover, as discussed previously, during the assembly process, the applied color-plus-clear coating can include surface defects in the clear coat surface which requires repair. Some automobile manufacturers elect to remove the defect and recoat the repair area with the same clear coat composition. In this instance, the cured clear coat must have excellent intercoat adhesion to the subsequently applied clear coat. It is known, however, that some clear coats when cured have poor intercoat adhesion with the subsequently applied repair clear coat.
In view of the foregoing, there remains a need in the coating industry for coating compositions which have improved properties such as acid etch resistance and mar and scratch resistance while maintaining excellent intercoat or interlayer adhesion to subsequently applied coatings and/or adhesives.
Also, many adhesion promoters are known in the art. Such adhesion promoters include, for example, phosphatized epoxy compounds, for example, the reaction product formed from phosphoric acid and a bisphenol A or hydrogenated bisphenol A diglycidyl ether. Typically, such adhesion promoters are useful for promoting adhesion of coating layers which contain them to a substrate, for example, a metallic substrate or an elastomeric substrate or to a previously applied coating layer. Also, such adhesion promoters can be used advantageously to promote cohesive integrity within a coating layer, for example, the cohesive integrity of a metal flake-containing basecoat. Further, it is known that adhesion promoter compositions, such as a phosphate wipe or an adhesion-promoting primer, can be topically applied to a cured coating to provide an adhesion promoting layer thereover, thereby improving adhesion of a subsequently applied coating. This, however, necessitates an additional and costly coating step in the coating application process. It is not known, however, to include an adhesion promoter as a component in a coating composition which will migrate during a curing reaction through the surrounding polymeric matrix to the surface of the resultant coating thereby promoting the interlayer or intercoat adhesion between the resultant coating and a subsequently applied coating.
As mentioned above, the surface of a coating can be modified by the inclusion of one or more surface active agents, for example, silicone oils, siloxanes, and fluorsurfactants, in the coating compositions to improve such properties as slip and mar resistance of such coatings. Typical surface active agents have solubility parameters or surface energies which are sufficiently different from the coating compositions (i.e., the composition without the surface active agent) such that, when included in the composition, the surface active agent can migrate or partition to the surface region of the cured coating as the composition cures. That is, the surface active agent is present at the surface region of the resultant coating layer. While such surface-modified coatings can exhibit improved slip and mar resistance, they often are difficult to recoat. Hence, the interlayer or intercoat adhesion with a subsequently applied coating is poor, sometimes resulting in delamination.
It has now been found that by selecting adhesion promoting components and surface active agents such that the solubility parameter of the coating composition containing both the adhesion promoting component and the surface active agent is sufficiently different from that of an analogous coating composition which does not contain the adhesion promoting component and the surface active agent, that the adhesion promoting component partitions to the surface region of the resultant coating. This can result in a concentration of the adhesion promoting component at the surface region which is greater than the concentration in the interior or bulk region of the coating layer. This partitioning effect of the adhesion promoting component can significantly increase its effect in promoting the adhesion of the coating layer which contains the adhesion promoter to a subsequently applied coating layer, as well as to the substrate to which it is applied.